Crepi­tus of the Knee: Creaky Bones or Some­thing More?

IT’S NOT un­usual for peo­ple to hear a noise or feel a crack­ing, crunch­ing, or pop­ping sen­sa­tion when they move their knee. This crack­ing or pop­ping sen­sa­tion, known as crepi­tus, is usu­ally due to air bub­bles be­ing caught in body tis­sues. It can hap­pen in the chest or the knee. In the knee, it can cause a sound when the knee is ex­tended.

Knee crepi­tus can hap­pen at any age, but it is more com­mon as peo­ple get older. It can af­fect one or both knees. The sound may be au­di­ble to other peo­ple, or it may not. Crepi­tus is of­ten harm­less, but if it hap­pens af­ter a trauma or if there is pain and swelling, it may need med­i­cal at­ten­tion. The three bones in the knee joint are the thigh­bone ( fe­mur), the shin­bone ( tibia), and the kneecap ( patella). The kneecap rests in a groove of the thigh­bone, called the trochlea. When a per­son bends or straight­ens their knee, the patella moves back and forth in­side this groove.

Two wedge- shaped pieces of car­ti­lage be­tween the thigh­bone and the shin­bone are called the menis­cus. These en­able the bones to glide smoothly against each other. The car­ti­lage is tough and rub­bery, and it helps to cush­ion the joint and keep it sta­ble. There is also a thin layer of tis­sue called the syn­ovial mem­brane that cov­ers the joints and pro­duces a small amount of syn­ovial fluid, which helps to lu­bri­cate the car­ti­lage.

The un­der­side of the kneecap is lined with car­ti­lage. This car­ti­lage “rubs” against the end of the fe­mur in the trochlear area and with ab- nor­mal wear can cause grind­ing ( crepi­tus). In most cases, the pop­ping sound comes from air seep­ing into the soft tis­sue, find­ing its way into the area around the joint and caus­ing tiny bub­bles in the syn­ovial fluid. When a per­son bends or stretches their knees, the bub­bles can burst with a pop­ping or crack­ing sound. While it may sound alarm­ing, this is harm­less.

How­ever, crepi­tus can also hap­pen as car­ti­lage rubs on the joint sur­face or other soft tis­sues around the knee when the joint moves, and when the car­ti­lage be­comes thin and wears away. If there is pain as the knee snaps or catches, it can be be­cause scar tis­sue, a menis­cus tear, or a ten­don is mov­ing over a pro­trud­ing bone within the knee joint.

Pain or swelling can be a sign of a more se­ri­ous prob­lem, such as patellofemoral pain syn­drome, torn car­ti­lage or other soft tis­sue, or os­teoarthri­tis ( OA). These is­sues may need med­i­cal at­ten­tion. Let’s look at them now in more de­tail. When the pres­sure be­tween the kneecap and the fe­mur is greater than usual, the car­ti­lage in the joint can start to soften and wear away, los­ing its smooth­ness and lead­ing to a con­di­tion called patellofemoral pain syn­drome ( PFS), or “run­ner’s knee.”

Rig­or­ous ex­er­cise such as jog­ging, squat­ting, and climb­ing stairs can put strain on the area be­tween the fe­mur and the kneecap joint. A sud­den in­crease in phys­i­cal ac­tiv­ity, such as ex­er­cis­ing more fre­quently, or run­ning fur­ther or on rougher ter­rain than usual, can also cause it.